config: add docs for ignoring all text below in the editor
This is an example of how to use the new skip-from-there string for ignoring the
diff in a commit message.
--- a/mercurial/help/config.txt Sat Dec 31 15:36:36 2016 -0600
+++ b/mercurial/help/config.txt Wed Jan 04 22:32:42 2017 -0600
@@ -418,6 +418,28 @@
"HG: removed {file}\n" }{if(files, "",
"HG: no files changed\n")}
+``diff()``
+ String: show the diff (see :hg:`help templates` for detail)
+
+Sometimes it is helpful to show the diff of the changeset in the editor without
+having to prefix 'HG: ' to each line so that highlighting works correctly. For
+this, Mercurial provides a special string which will ignore everything below
+it::
+
+ HG: ------------------------ >8 ------------------------
+
+For example, the template configuration below will show the diff below the
+extra message::
+
+ [committemplate]
+ changeset = {desc}\n\n
+ HG: Enter commit message. Lines beginning with 'HG:' are removed.
+ HG: {extramsg}
+ HG: ------------------------ >8 ------------------------
+ HG: Do not touch the line above.
+ HG: Everything below will be removed.
+ {diff()}
+
.. note::
For some problematic encodings (see :hg:`help win32mbcs` for